The present invention relates generally to power electronic circuits, and more particularly to power electronic circuits that include snubbers.
Power electronic circuits are, in generalized terms, circuits that use electronic components to perform operations in the field of power conversion. One subset of power electronic circuits operate using an DC power source. This subset includes DC motors and DC-DC or DC-AC converters. DC motors include both brush-type motors and brushless motors. Such devices experience widespread use. DC-DC converters include boost and buck type converters, which are also in widespread use.
The power electronic circuits that operate using a DC power source are often employed in vehicular applications and portable applications. For example, DC brushless motors are often used in radiator fans in automobiles and trucks. The DC power source in such application is the vehicle battery.
One potential issue with DC-powered circuits, including DC power electronic circuits, relates to the behavior of such circuits when the polarity of the DC power source is inadvertently reversed. In particular, DC power sources, for example, batteries, are often replaced by consumers and/or mechanics having modest electrical training. As a result, batteries are occasionally installed incorrectly, and more specifically, such that they are connected in reverse polarity. Because the possibility of the reverse polarity of the DC power source exists, it is desirable to protect the components of the power electronic circuit from damage or destruction upon occurrence of an accidental reverse wiring of the DC power source.
To this end, power electronic circuits may include a reverse polarity protection arrangement. The reverse polarity protection arrangement substantially limits reverse polarity DC current from flowing through portions of the power electronic circuit that could otherwise be damaged by such reverse polarity current.
One design of a reverse polarity protection arrangement consists of a MOSFET switch coupled in series between the DC power source and the power electronic circuit. For example, U.S. Pat. No. 4,857,985 to Miller and U.S. Pat. No. 5,757,600 to Kiraly show reverse polarity protection arrangements in which the negative terminal (ground) connections of an electronic circuit are connected to the negative terminal of the battery source through the source-drain junctions of a MOSFET switch. The circuits are arranged such that as long as the battery is properly connected, the gate voltage of the MOSFET switch is high enough to turn the switch on (i.e. allow the switch to conduct drain to source). However, if the battery is connected such that it is in reverse polarity, the gate voltage cannot provide a sufficient turn on voltage. As a result, the reverse polarity energy cannot reach the electronic circuit.
One problem associated with the reverse polarity protection arrangements of U.S. Pat. Nos. 4,857,985 and 5,757,600 arises from the location of the MOSFET in the return or ground path. Location of the MOSFET in the return or ground path introduces a stray inductance into the return/ground path, which is undesirable with respect to suppressing electromagnetic radiation.
A solution that does not introduce a stray inductance into the return path of the electronic circuit is provided in U.S. Pat. No. 5,642,251 to Kopera, Jr. et al and U.S. Pat. No. 5,517,379 to Williams et al. Those patents employ reverse polarity protection arrangements in which a MOSFET switch is series connected source to drain between the positive supply lead of the DC power source and the positive supply terminal of the electronic circuit. The arrangements, similar to those described above, generate a gate voltage sufficient to allow current to flow source to drain (and thus through the electronic circuit) only when the DC power supply is properly connected. One problem with the solutions of U.S. Pat. Nos. 5,642,251 and 5,517,379 is that they require a gate voltage that is higher than the DC power source voltage in order to turn on the MOSFET switch. To this end, a charge pump circuit or the like must be employed to generate the gate voltage. Although the charge pump circuit generates sufficient voltage to turn on the MOSFET, the charge pump circuit is a direct drain on the DC power source and also requires significant amounts of space and power consuming circuitry.
What is needed therefore, is a reverse polarity protection arrangement that avoids shortcomings of the prior art. In particular, there is a need for a reverse polarity protection arrangement that does not introduce a stray inductance into the ground path of the circuit while avoiding excessive power consumption as is required to operate a special charge pump circuit.
The present invention fulfills the above needs, as well as others, by providing a reverse polarity protection arrangement that includes a protection switch coupled in the positive voltage supply line that is configured to be powered by a snubber circuit of a power electronic circuit. Snubber circuits in power electronic circuit topologies are circuits that absorb surge voltages that occur when an inductive element in the power electronic circuit is suddenly turned off. The snubber circuit stores the surge voltage, which is in excess of the supply voltage. In accordance with the present invention, the snubber circuit voltage is used to bias the protection switch when the DC power source is properly connected. Exemplary power electronic circuits that include such snubber circuits include DC brushless motor control circuits and boost-type DC-DC switching power converters.
An exemplary embodiment of the present invention is a protection circuit for use in a power electronic circuit having a positive supply input, a negative supply input, and a snubber. The protection circuit includes a snubber bias lead, a positive voltage lead, a positive supply lead, a switch and a reverse polarity shut off circuit. The snubber bias lead is adapted to be coupled to obtain a bias voltage from the snubber of the power electronic circuit, the positive voltage lead is adapted to be coupled to a positive voltage lead of a power supply, and the positive supply lead is adapted to be coupled to the positive supply input of the power electronic circuit. The switch has a control input coupled to the snubber bias lead, a first terminal coupled to the positive voltage lead, and a second terminal coupled to the positive supply lead. The switch is operable to selectively allow current flow from the second terminal to the first terminal based at least in part on the bias voltage at said control input. The reverse polarity shut off circuit is coupled to the negative supply input and the control input, the reverse polarity shut off circuit operable cause the switch to inhibit current flow from the second terminal to the first terminal when a positive supply voltage is detected at the negative supply input.
In the embodiments described herein, a current limiting device may furthermore be coupled between the snubber bias lead and the control input of the switch. The current limiting device does not affect normal operation, but ensures that a low impedance, high current path is not provided through the snubber in a reverse polarity situation. The current limiting device may suitably be a resistance that substantially exceeds the normal impedance of the coil circuits.
The present invention thus provides a method and apparatus of protecting a power electronic circuit from excessive reverse polarity current when a DC power source is connected to the electronic circuit in reverse polarity without introducing a stray inductance into the negative supply line (i.e. ground) and without consuming power directly from the DC power source.